1. Field of the Invention
The present invention relates to a photoelectric copying machine and, more particularly, to an apparatus and method for discharging the photoconductive layer made of a selenium alloy, or the like, to provide a constant and stable initial condition of the photoconductive layer at the beginning of each copying operation, thereby providing a uniform copied paper at any time during the series of operations.
2. Description of the Prior Art
When a photoconductive plate or layer made of a selenium alloy, or the like, is subjected to repeated cycles of charging, exposure and discharging, an effect known as fatigue is encountered. This effect is manifested as an increase in the dark decay rate of the plate potential, i.e., decrease charge retention. Fatigue in amorphous selenium is caused by the build-up, within the photoconductive film, of trapped charges which produce a high field condition at the interface between the film and the conductive substrate. These internally trapped charges also produce an increased residual potential.
If a fatigued photoconductive layer is allowed to rest in the dark it will gradually return to its original normal condition. Fatigue is not an important factor when the period between use cycles is long, or in cases where several plates can be used in rotation. However, in rapid cycling, the effect of fatigue may become troublesome, producing poor copies, such as copies with insufficient darkness of the dark lines, copies with a blur on the white areas, or copies with a ghost image, which is an image remaining from the previous copying operation. This problem becomes more and more serious when the photoconductive layer is made of As.sub.2 Se.sub.3 having a high sensitivity to light. Also, since the fatigue progresses logarithmically during the series of copying operations, the condition of the imaged copy paper is different from those obtained at the beginning and those obtained at the end. Thus, it has been difficult to obtain a uniform condition for all of the copy papers obtained during a series of copying operations. The above is discussed in detail in a book "ELECTROPHOTOGRAPHY" written by R. M Schaffert.
Various methods and devices have been proposed to overcome the problems described above, one of which is disclosed in U.S. Pat. No. 3,511,649 issued to E. J. Felty et al in which a light source producing lights having a waveform shorter than 5400 angstrom is employed to prevent th effect of fatigue. However, in the case where the high sensitive plate As.sub.2 Se.sub.3 is operated at rapid cycling, this arrangement results in insufficient discharge of the plate, regardless of an increase of the light amount, thereby resulting in the increase of the residual voltage which provides blur on the white areas.
Another prior art device is disclosed in Japanese Patent Publication (unexamined) No. 53-148444 of Xerox Corp. published Dec. 25, 1978. This patent publication is based on U.S. patent application Ser. No. 801,115 filed May 27, 1977. According to this reference, a pre-fatigue system is employed such that the photoconductive plate is exposed, before a series of copying operations, so as to receive light having a peak at 6200 angstrom. Thus, the fatigue in the plate is progressed intentionally to a certain degree before the first copying operation. Although this arrangement may give a stable initial corona charge during the first number of cycles of the copying operation, the residual voltage may increase after the number of cycles of the copying operation are carried out serially. Thus, the problem of obtaining a uniform condition for all the copy papers during a series of copying operations still exists.
Yet, another prior art device is disclosed in Japanese Patent Publication (unexamined) No. 58-114082 published July 7, 1983. According to this reference the discharging step and the corona charging step are carried out with a time interval of more than 0.2 second, which is sufficiently long to neutralize the trapped charges within the photoconductive layer. When this arrangement is employed, the photoconductive layer will not be fatigued, thereby providing a uniform condition for all the copy papers. However, with this arrangement, it is not possible to realize the rapid cycling of the copying operation.
In addition to the above, the photoconductive layer is susceptible to temperature such that the dark decay rate changes with respect to the temperature change. This change is caused by the change of the number of charges trapped within the photoconductive layer with respect to temperature. Thus, when temperature changes, there arises the same problems as discussed above, such as the insufficient darkness of the dark liens, blur in the white areas, or production of a ghost image.
To overcome the above problem caused by the temperature change, various methods and devices are proposed. One such solution is disclosed in Japanese Patent Publication (examined) No. 55-40971 in which a temperature detector is provided for detecting the temperature of the photoconductive layer. The bias voltage for developing the latent image is changed with respect to the temperature. However, this method has the problem such that the center portion of the dark areas will not be as dark as the peripheral portion.